The present invention is directed to laser technology. It is concerned particularly with the production of short pulses of laser light.
One approach that has been employed in the past for obtaining ultra-short laser pulses is to employ a multi-longitudinal-mode laser. Such a device takes advantage of a plurality of the modes of the laser so as to produce a composite beam of a plurality of different frequencies. If those frequencies are evenly spaced, they will reinforce periodically, with a period equal to the reciprocal of the frequency spacing, to produce pulses even though each mode operates continuously. Such a device can produce pulses at extremely high pulse-repetition rates.
Unfortunately, the multi-mode approach is afflicted with a number of practical difficulties. In the first place, the multi-mode laser is itself relatively expensive. Also, the natural modes are not evenly spaced, and considerable effort must be taken to "drive" them into proper synchronism. That is, the action of the mode locker, whose function is to keep the different modes in the proper relative phases, is resisted by the tendency of the modes to revert to their natural frequencies and thus fall out of the proper phase relationship.
Another problem is that the lasing medium manifests nonlinear effects that cause the modes to interact. Specifically, lasing at one mode tends to reduce the gain at another mode, so the relative amplitudes of the different modes cannot be readily controlled and the bandwidth of the multi-mode laser is effectively reduced. As a consequence, it is difficult to control the pulse shape.